1. Field of the Invention
The present invention relates to an apparatus and a method for estimating whether or not the friction coefficient of a road surface is low or the friction coefficient itself of the road surface while a vehicle is running.
The present invention further relates to an apparatus and a method for controlling a driving condition of a vehicle in accordance with the estimated friction coefficient of the road surface on which the vehicle is running, so that a stable running of the vehicle can be obtained.
2. Description of the Related Art
Detection of the friction coefficient of a road surface is essential to stable and smooth drive of a vehicle. If the road on which the vehicle is running changes from a dry ordinary road into a snow-covered or frozen road, the friction coefficient of the road surface suddenly lowers. When the vehicle runs on such a low-friction road, that is, a so-called low-.mu. road, the driving wheels of the vehicle are very liable to slip, so that it is extremely difficult to control the vehicle. If the driving wheels undergo a slip, a driver is expected to reduce the depth of depression of the accelerator pedal, thereby controlling the engine output, in order to stop the slip. It is very hard for even a skilled driver, however, to control the engine output by only adjusting the depth of depression of the accelerator pedal.
When the vehicle turns or circles, on the other hand, a centrifugal force or transverse acceleration acts on the vehicle at right angles to the running direction thereof. If the running speed of the vehicle is too high, therefore, the transverse acceleration may exceed the gripping force of tires, possibly entailing a skid of the vehicle. Such a skid of the vehicle tends to be caused especially when a gateway of the turning road cannot be found or when the radius of curvature of the turning road gradually decreases. This is because the driver cannot properly control the engine output or vehicle speed with ease in such a situation.
A conventional vehicle whose turning characteristic is represented by the so-called understeering performance cannot stably pass a turning road unless the driver additionally turns the steering wheel as the transverse acceleration acting on the vehicle increases while the vehicle is turning or circling. If a specific value for the vehicle is exceeded by the transverse acceleration acting thereon, however, the vehicle suffers a skid despite the additional turn of the steering wheel, so that the actual turning track of the vehicle deviates a target turning track which is imaged by the driver. This tendency is particularly noticeable in front-engined vehicles of the fron-wheel drive type whose turning characteristic is represented by a positive understeering performance.
In consideration of these circumstances, there are proposed various output control devices which can compulsorily lower the engine output, without regard to the depth of depression of the accelerator pedal by the driver, if the driving wheels suffer a slip or if the transverse acceleration acting on the vehicle increases to the turning limit. To effectively enjoy the functions of the output control devices of this type, the timing for the start of operation of the devices must be properly controlled.
However, the timing for the start of operation of the output control devices, that is, the point of time when a slip of the driving wheels begins or when the turning limit is reached by the transverse acceleration of the vehicle, greatly varies depending on the friction coefficient of the road surface on which the vehicle is running, that is, on whether or not the running road is a low-.mu. road. For these output control devices, therefore, it is very important properly to detect the friction coefficient itself of the road surface or whether or not the running road is a low-.mu. road. Meanwhile, the friction coefficient of the road surface is conventionally detected or measured by various methods. However, none of these conventional methods are suited for the output control devices of this type.
As described above, the conventional output control devices, which are used for the turning control of the vehicle, are expected properly to detect the friction coefficient of the road surface or determine whether the running road is a low-.mu. road, even while the vehicle is turning. There are no conventional methods, however, which fulfill these requirements.